Super speed because it automatically gives you durability and if you are faster than your opponent you can’t lose at all ever. The only ability that might be better than that is the ability to launch large locomotive objects.

Actually it would make a genuinely interesting Elseworlds if Batman dipped into more straightforward Wuxia, and trained to develop supernormal powers that were entirely dependent on his will power, right mindedness, creativity and ingenuity.

Chi power, similar to Iron Fist or Karate Kid. That would be a good choice. But I like another option too. It's tech based, would be to equip in his standard suit the possibility to increase his strength (up to 15-20 tons level) and endurance. Both of these things however, according with his philosphy, would be used just in extreme circumstances, just when he already pushed himself to the limit, he always does. But, yep, definitely, Chi power would be the best level up for him.

Super speed because it automatically gives you durability and if you are faster than your opponent you can’t lose at all ever. The only ability that might be better than that is the ability to launch large locomotive objects.

The power to instill an irrational sense of fear in others, not just because of him being intimidating or his reputation, but a power where he outright causes them to go crazy with fear. He could use it via Batglare.

How about at some point before the formation of the Justice League Bruce discovered a new element called Plotium and surgically placed it inside himself or something and the comic explains that it's a "highly unstable" material with "reality warping abilities" that becomes an excuse for plot contrivances or beating up characters he really has no business fighting?

Actually, he doesn't even need to place it in there. A chunk of Plotium could just teleport in and there you go! It's perfect.